waterbird vs aquatic bird

waterbird and aquatic bird both are nouns.

Word	Noun	Adjective	Verb	Adverb
waterbird	Yes	No	No	No
aquatic bird	Yes	No	No	No

As nouns, aquatic bird is a hypernym of waterbird; that is, aquatic bird is a word with a broader meaning than waterbird:

waterbird: freshwater aquatic bird
aquatic bird: wading and swimming and diving birds of either fresh or salt water

waterbird (noun)	aquatic bird (noun)
freshwater aquatic bird	wading and swimming and diving birds of either fresh or salt water